The most common and most likely answer to this crossword clue is the 11 letter word ABECEDARIAN
A
B
E
C
E
D
A
R
I
A
N
More clues leading to the result ABECEDARIAN
Buddy explains!

The straight part
We know that "rudimentary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rudimentary = abecedarian.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ABECEDARIAN mean?
ABECEDARIAN noun- Someone who is learning the alphabet.
- An elementary student, a novice; one in the early steps of learning.
- Someone engaged in teaching the alphabet; an elementary teacher; one that teaches the methods and principles of learning.
- A work which uses words or lines in alphabetical order.
- Pertaining to someone learning the alphabet or basic studies; elementary; rudimentary.
- Pertaining to the alphabet, or several alphabets.
- Arranged in an alphabetical manner.
- Relating to or resembling an abecedarius.